Connect to the top of the first stitch of the last row with a slip stitch. Ch2, turn, DC into the 4th loop from your hook. Basically, you need to attach the two ends of the row together, build up height for the first stitch of the next row, then go back the other way without adding stitches.

When you get to the end of that row, do it the same except only Ch1 after you turn and SC into the 3rd loop from your hook.